1. Field of the Invention
The present invention relates to a touch panel and a display apparatus having the touch panel.
2. Description of the Background Art
The touch panel function is conventionally known and receiving attention as one of good interface means, where a touch by, e.g. a finger, on the touch panel is detected and the coordinates of the touched position are specified for input operation. Such touch panels are manufactured as products using various schemes for detecting the position of touch by, e.g. a finger, such as resistive film schemes and capacitive schemes, for example.
The PCT (Projected Capacitive Touch screen) scheme is known as one of the capacitive schemes, where the front side of a touch screen having a touch sensor is covered by a protective plate, e.g. a glass plate having a thickness of several millimeters, such that a touch by, e.g. a finger on the protective plate can be detected (for example, refer to Japanese Patent Application Laid-Open No. 9-511086 (hereinafter referred to as Patent Document 1), from page 7 line 19 to page 8 line 4, same page line 23 to page 9 line 6, page 13 lines 4 to 12, and FIGS. 1, 2 and 8). The PCT scheme is advantageous in that the protective plate provided on the front side improves robustness, that it can even detect a touch by a gloved finger, and that it has no movable parts and so has a long life.
The touch screen of a touch panel using the PCT scheme described in Patent Document 1 has a first series of conductive material patterns (conductor elements) formed of a thin conductive film as sensing conductors for sensing capacitance, and a second series of conductive material patterns, with an insulating film formed therebetween. There is no electrical contact between the conductive material patterns, and the first series of conductive material patterns and the second series of conductive material patterns form a plurality of intersections. The conductor elements for sensing capacitance are connected to a capacitance controlled oscillator through an output line and a multiplexer. The output is counted by a divider to provide capacitance sensing data.
Materials most suitable as the conductive material are metal materials such as silver. On the other hand, since a touch panel is mainly used with a display apparatus, the visibility of the conductive material patterns can be a problem as a display characteristic. Therefore, transparent conductive films, such as indium oxide (ITO), may be used in order to reduce the visibility of the conductive material patterns. Also, conductive thin lines, several to 20 μm thick, may be used in place of conductive material patterns.
The capacitance controlled oscillator of such a touch panel can be a relaxation oscillator or a hysteresis oscillator. The oscillation cycle of such an oscillator is generally determined by the charge/discharge time constants of resistive and capacitive elements, and the capacitive elements are structured to include the capacitance formed between the sensing lines (conductor elements) and finger (hereinafter also referred to as touch capacitance). Then, when a finger of a user touches, the oscillation cycle of the oscillator varies according to the touch capacitance formed between the sensing lines and finger, and the amount of variation is detected to recognize the touch and its position.
For upsizing of such touch panels, it is required to obtain good sensing accuracy by reducing parasitic capacitance or line resistance occurring in the touch panel, and to obtain good display quality.
In touch panels in which the sensing lines are made of a transparent conductive film, the display quality of the display apparatus provided with the touch panel is not likely to be degraded due to the visibility of the sensing lines. However, such touch panels are likely to suffer reduced sensing accuracy because the transparent conductive film forming the sensing lines generally has high resistance value. Accordingly, it was difficult to upsize touch panels in which the sensing lines are made of a transparent conductive film.
On the other hand, in touch panels in which the sensing lines are made of a metal film good sensing accuracy is obtained even when the touch panels are upsized, because the line resistance is lower than that of sensing lines made of transparent conductive film.
However, when the sensing lines are formed of plain lines, the large areas of the lines degrade visibility. Accordingly, when the sensing lines are formed of a film, it is desirable to form each sensing line as a bundle of less visible thin lines having a line width of about several to 20 μm.
However, when the sensing lines are formed of bundles of thin lines, the resistance value is increased higher than when they are formed of plain lines. For example, when a length of a plain line is L, a width of the plain line is W1, and sheet resistance value of the plain line is ρ, and a length of a bundle of thin lines is L, a width of one thin line is W2, the number of thin lines is N, and sheet resistance value of each thin line is ρ, then the relation W2×N<W1 holds between the line widths per one sensing line. Also, the resistance R1 of the plain line is: R1=L/W1×ρ, and the resistance R2 of a bundle of thin lines, which is the parallel resistance of N thin lines, is: R2=(L/W2×ρ)/N. That is to say, from the expressions, the relation between the resistance values of plain line and bundle of thin lines is: R2>(L/W1×ρ), and it is seen that the resistance value of bundles of thin lines is higher than that of plain lines.
Also, because individual thin lines are formed thinly, they are likely to be influenced by abnormalities during pattern formation etc. When a pattern defect occurs due to some process abnormality, the defective pattern cannot contribute to the sensing operation and then the sensibility is considerably reduced. For example, when a sensing line is formed of three thin lines and one of them cannot contribute to the sensing operation due to a pattern defect caused by some process abnormality, then the sensibility is reduced to ⅔. This also reduces yield.